As an example of a conventional radio communication method, there is a multi-carrier modulation/demodulation method as typified by an OFDM (orthogonal frequency division multiplexing) method and a DMT (discrete multi-tone) method. The multi-carrier modulation/demodulation method is used, for example, in a wireless LAN or an ADSL, and is a method of transmitting a plurality of frequencies by allocating the frequencies to orthogonal carriers. In the method, a guard interval or a cyclic prefix is used to eliminate an effect of a delayed wave (GB) caused by, for example, a condition of a transmission path between a transmitter and a receiver. The receiver eliminates the effect of the delayed wave in the guard interval, for example, by applying an FFT to an OFDM symbol from which the guard interval is removed, and thereby demodulating data.
In the OFDM method, when a delayed wave exceeding the guard interval arrives, an intersymbol interference is generated, and thus overall characteristics is degraded significantly. The above problem can be solved by adding a guard interval longer than a delay time estimated by the transmitter side. However, with this process, an overhead of the guard interval increases, and thereby decreasing a transmitting efficiency.
In a cellular system, for example, if a terminal is located far from a base station, a delay time is lengthened. On the other hand, if the terminal is located near the base station, the delay time is shortened. In other words, in the cellular system, a length of an appropriate guard interval depends on a location of the terminal or an environment.
An example of a conventional technology for a case where the delayed wave exceeding the guard interval arrives or a case where a length of the appropriate guard interval depends on a location of the terminal or an environment, as described above, is “an OFDM communication system and an OFDM communication method” disclosed in patent document 1. In the technology disclosed in patent document 1, a guard interval length, which is used to be a fixed length before, is adaptively controlled depending on a multipath delay time, and thereby achieving the maximum transmitting efficiency.    [Patent document 1] Japanese Patent Application Laid-Open No. 2002-374223